Demand and conversion
Two mirror metrics for whether a visit went anywhere — and the definition changed underneath everyone when analytics moved to GA4.
In the older model a bounce was a session with exactly one interaction, which meant a visitor who read a page for four minutes and left satisfied was counted identically to one who arrived by mistake. In GA4 the primary metric is engagement: a session counts as engaged if it lasts over ten seconds, fires a conversion, or has two or more page views. Bounce rate is simply its complement.
The practical consequence is that bounce figures from before and after the migration are not comparable, and a reported improvement across that boundary is usually the definition changing rather than the site.
On a page that answers one question — opening hours, a phone number, a definition — a visitor who leaves immediately was served perfectly. Judge single-answer pages on whether the answer was found, not on whether a second page was loaded.
The definitions are the easy part. Whether the figure on your dashboard was computed this way is a different question, and usually the more expensive one.